1. Field of Invention
The present invention relates to an optical interference type of color display. More particularly, the present invention relates to an optical interference type of color display having an improved color shift and contrast ratio (CR).
2. Description of Related Art
At present, lightweight and slim flat panel displays such as liquid crystal display (LCD), organic light-emitting device (OLED) or plasma display panel (PDP) are widely adopted in our everyday life. In particular, liquid crystal panels have become one of the mainstream displays. However, most LCD still has a number of drawbacks including narrow visual angle, moderate response time, need for a color filter for full coloration, and need for a polarizer leading to a poor optical utilization of light source and energy wastage by a back light module.
To improve the operating efficiency of LCD, a new type of LCD called an optical interference display is developed. The optical interference panel comprises an array of optical interference modulators. Each optical interference modulator includes a transparent electrode, a reflective electrode and a support layer for supporting the reflective electrode. Through the support of the support layer, an air gap with a specified thickness is formed between the reflective electrode and the transparent electrode. Light entering from the transparent electrode of the optical interference modulator passes through the air gap and impinges upon the second electrode. Light impinging the second electrode is reflected back to emerge from the modulator through the transparent electrode. Because light passing through air gap of different thickness may result in different degree of optical interference, different colors are produced. For example, red light, green light and blue light can be produced in this way. In addition, the design of the reflective electrode inside the optical interference modulator must integrate with a micro-electromechanical system (MEMS) so that the optical interference modulator can switch between an “on” or an “off” state to illuminate or darken a spot on the panel.
The aforementioned optical interference modulators inside the optical interference display need no additional coloring filter or polarizer for producing a suitable color point and hence able to save some production cost. In addition, each optical interference modulator consumes very little electric power, quick to respond to electrical signals and operates in a bi-stable state. Therefore, the optical interference display is suitable for low power consumption products including most portable device such as mobile phone, personal digital assistant (PDA), electronic book (e-book) and so on.
FIG. 1 is a schematic sectional view of a conventional optical interference color display structure. As shown in FIG. 1, the optical interference color display 100 mainly comprises a transparent substrate 110, a patterned support layer 120, a plurality of first electrodes 130, a plurality of optical films 140 and a plurality of second electrodes 150. In general, the transparent substrate 110 is a glass substrate or a substrate made from a transparent material. The patterned support layer 120 is positioned on the transparent substrate 110 for supporting the edges of the second electrodes 150. The first electrodes 130 are also positioned on the transparent substrate 110. The first electrodes 130 are transparent electrodes fabricated using a material including indium-tin-oxide (ITO). The optical film 140 is positioned on the first electrodes 130. Typically, the optical film 140 is a composite stack having a multiple of alternately positioned high dielectric constant films and low dielectric constant films. The second electrodes 150 are positioned over the first electrodes 130. Through the support of the patterned support layer 120, the second electrodes 150 are positioned over the first electrodes 130. The second electrodes 150 are typically fabricated using a highly reflective metallic material.
In general, a conventional optical interference color panel comprises a plurality of optical interference modulators each having a different air gap thickness. As shown in FIG. 1, the air gap between the second electrode 150 and the first electrode 130 is different for different optical interference modulators. To produce color light, the optical interference modulators are designed to have three different air gap separations d1, d2 and d3. The optical interference modulator with an air gap separation of d1 emits red light; the optical interference modulator with an air gap separation of d2 emits blue light; and, the optical interference modulator with an air gap separation of d3 emits green light. In other words, as light coming from outside penetrates through the transparent substrate 110, the first electrodes 130 and the optical films 140, the light needs to pass through different air gap thickness d1, d2, d3 before arriving at the respective second electrodes 150. Thereafter, the light emerges from the transparent substrate 1100 after reflecting back by the second electrodes 150. Due to different degree of interference at different air gap thickness, red light, green light and blue light are produced.
In a conventional optical interference modulator, the second electrode 150 must be fabricated using a reflective material with good mechanical properties. When the second electrode 150 and the first electrode 130 are coupled to a bias voltage, the second electrode 150 may shift towards the first electrode 130 due to electrostatic attraction. Any movement of the second electrode 150 may lead to a slight variation of the air gap d1, d2 and d3. Through a slight change in the thickness of the air gaps d1, d2, and d3, various optical interference modulators (pixels) within the display can be switched to an “on” or an “off” state.
In the optical interference color display 100, images on display may be affected by user's viewing angle due to an intensification of color shift and a deterioration of contrast ratio. Thus, the conventional technique often demands the attachment of an optical diffusion plate 160 to the outer surface of the transparent substrate 110 for improving color shift and contrast ratio. However, the attachment of an optical diffusion plate not only increases the overall thickness of the color display 100 (an additional thickness of about 2 mm), but also increases material cost.